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Industrial Sensors Industrial Sensors• Proximity
– Mechanical– Optical– Inductive/Capacitive
• Position/Velocity– Potentiometer– LVDT– Encoders– Tachogenerator
• Force/Pressure• Vibration/acceleration
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Definitions Definitions • Accuracy: The agreement between the actual
value and the measured value• Resolution: The change in measured variable
to which the sensor will respond• Repeatability: Variation of sensor
measurements when the same quantity is measured several times
• Range: Upper and lower limits of the variable that can be measured
• Sensitivity and Linearity
Proximity SensorsProximity Sensors• Widely used in general industrial automation
– Conveyor lines (counting,jam detection, etc)– Machine tools (safety interlock, sequencing)
• Usually digital (on/off) sensors detecting the presence or absence of an object
• Consist of:– Sensor head: optical, inductive, capacitive– Detector circuit– Amplifier– Output circuit: TTL, solid state relay
Mechanical Proximity SwitchesMechanical Proximity Switches• Essentially a
mechanical switch• On/off operation only• Two general modes
– Normally Open (NO)– Normally Closed (NC)
• Come in a wide variety of mechanical forms
• For a wide range of uses
Actuator
Common
NormallyClosed
NormallyOpen
Example Mechanical Example Mechanical Proximity Switches Proximity Switches
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When to Use Mechanical When to Use Mechanical Proximity SwitchesProximity Switches
• Where physical contact is possible• Where definitive position is required• In operation-critical or safety-critical
situations• Where environment conditions preclude
the use of optical or inductive sensors
Applications and Use of Applications and Use of Mechanical Proximity SwitchesMechanical Proximity Switches
• Easy to integrate into machinery of all types
• Requires contact (thus wear)• Range of voltages: DC 0-1000V, AC, etc.• Very robust (explosion proof if required)• Usually used as:
– Limit switch– Presence/absence indicator– Door closed/open
Places You Find Mechanical Places You Find Mechanical Proximity Switches !Proximity Switches ! Optical Proximity SensorsOptical Proximity Sensors
• Consist of a light source (LED) and light detector (phototransistor)
• Modulation of signal to minimize ambient lighting conditions
• Various models: 12-30V DC, 24-240V AC, power• Output: TTL 5V, Solid-state relay, etc.
DemodulatorAmplifier
Modulator Power
Output
Mixer Signal
Power Supply
Load
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Operational ModesOperational Modes• Through Beam:
– Long range (20m)– Alignment is critical !
• Retro-reflective– Range 1-3m– Popular and cheap
• Diffuse-reflective– Range 12-300mm– Cheap and easy to use
Example Optical Proximity IExample Optical Proximity I
Optical Fibre Delivery System
Example Optical Proximity IIExample Optical Proximity II
Slot BeamSystems
When to use an Optical When to use an Optical Proximity SensorProximity Sensor
• Pros– Non-contact, no moving parts, small.– Fast switching, no switch bounce.– Insensitive to vibration and shock– Many configurations available
• Cons– Alignment always required– Can be blinded by ambient light conditions
(welding for example)– Requires clean, dust and water free, environment
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Applications of Optical Proximity Applications of Optical Proximity SensorsSensors
• Stack height control/box counting• Fluid level control (filling and clarity)• Breakage and jam detection• And many others…
http://www.omron-ap.com/application_ex/index.htmhttp://www.sick.de/english/products/products.htmhttp://content.honeywell.com/sensing/prodinfo/
Other Optical Devices Other Optical Devices
LightCurtain
Collision Detection
Ultrasonic Proximity SensorsUltrasonic Proximity Sensors• Use sound pulses• Measures amplitude and
time of flight• Range provides more than
on/off information• Frequencies 40KHz-2MHz
Pulse
Echo
Vibrating Membrane(metal or ceramic)
Sensor Object
When to use Ultrasonic SensorsWhen to use Ultrasonic Sensors
• Provide range data directly:• Level monitoring of solid and liquids• Approach warning (collisions)• Can (usually) work in heavy dust and
water• Ambient noise is potentially an issue
http://www.automationsensors.com/
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Example ApplicationsExample ApplicationsCar Wash Application
Paper roll Thickness Monitor
Waste water flow volume
Inductive and Capacitive Inductive and Capacitive Proximity SensorsProximity Sensors
• Inductive sensors use change in local magnetic field to detect presence of metal target
• Capacitive Sensors use change in local capacitance caused by non-metallic objects
• Generally short ranges only• Regarded as very robust and reliable
Example Inductive Sensors IExample Inductive Sensors I
Detection of open/close functions
Detection of rotation
Example Inductive Sensors IIExample Inductive Sensors II
Bulk mounted inductivesensors. Detect presence ofobject without contact.Range 3mm +/- 10%
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Example Capacitive SensorsExample Capacitive Sensors
Panel Mounted Capacitive Sensor. Can detect wood, plastic and metal. Range 3mm-25mm
Flat mounted CapacitiveSensor. Used for detecting panels of glass. Range=10mm +/- 10%
Position and Velocity SensorsPosition and Velocity Sensors• Position and velocity measurement is often
required in feedback loops• For positioning, and velocity control• Position measurement:
– Potentiometers– LVDT– Encoders
• Velocity Measurement:• Tachometer
Potentiometers Potentiometers
RVin
Vout
An analog sensorWorks as a voltage divider
Types of PotentiometerTypes of Potentiometer• Wirewound
– Wiper slides along coil of Ni-chrome wire– Wire tends to fail, temperature variations
• Cermet– Wiper slides on conductive ceramic track– Better than wire inmost respects
• Plastic film– High resolution– Long life and good temperature stability
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Linear PotentiometersLinear Potentiometers When to use a PotentiometerWhen to use a Potentiometer
• Pros– Require analog signal for control– Require absolute positional information– Low cost
• Cons– Temperature and wear variations– Not in dusty or wet environments
Linear Variable Differential Linear Variable Differential Transformer (LVDT)Transformer (LVDT)
• An LVDT consists of a magnetic core that moves in a cylinder
• The sleeve of the cylinder contains a primary coil that is driven by an oscillating voltage
• The sleeve also contains two secondary coils that detect this oscillating voltage with a magnitude equal to displacement
• The automatic nulling that can be achieved using two coils makes LVDTs very accurate (submillimetre)
Vinsinωt
Voutsin(ωt+φ)
Phase measurement
LVDT Signal ConditioningLVDT Signal Conditioning
• Uses AC modulation, demodulation and phase comparison
• Available in a single monolithic package
PowerSupply
CarrierOscillator
AmplitudeControl
LVDT
CurrentAmplifier Demodulator
PhaseShifter
ZeroSet
AC Power
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Example Example LVDTsLVDTs
Spring-loaded Standard for use In hydraulic cylinders
Free core LVDTs for use in hostile environmentsAnd total emersion
When to use an LVDTWhen to use an LVDT
• High accuracy• Linear operation (synchro resolver
is equivalent rotary LVDT)• Harsh environment• Analog position control • Embedding (in cylinder for
example)
Optical EncodersOptical Encoders• Encoders are digital Sensors commonly used to
provide position feedback for actuators• Consist of a glass or plastic disc that rotates
between a light source (LED) and a pair of photo-detectors
• Disk is encoded with alternate light and dark sectors so pulses are produced as disk rotates
Encoder Internal Structure Encoder Internal Structure
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Incremental EncodersIncremental Encoders• Pulses from leds are
counted to provide rotary position
• Two detectors are used to determine direction (quadrature)
• Index pulse used to denote start point
• Otherwise pulses are not unique
Absolute EncodersAbsolute Encoders• Absolute encoders
have a unique code that can be detected for every angular position
• Often in the form of a “grey code”; a binary code of minimal change
• Absolute encoders are much more complex and expensive than incremental encoders
Encoder processingEncoder processing• Need a squaring
circuit to digitise signal
• A counter and index monitor
• Generally available in monolithic form
• Often with algorithms for control externally programmable
When to Use an EncoderWhen to Use an Encoder• Require accurate position
information:– 10,000 line incremental– 360 line absolute
• Digital feed-back loop• Compact and reasonably
rugged (not as good as inductive)
• Linear encoders also available
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TachometersTachometers• Measurement of rotary
speed using a DC generator
• Essentially a motor running in reverse
• Used to be common to have these attached to motors to enable direct analog feedback
• Much less common now with digital control (use incremental encoders)
Tacho generator for large industrial plant (GE)
Force and Pressure Force and Pressure • Force and Pressure generally measured
indirectly through deflection of an alternate surface
• Mechanism include:– Physical motion and measurement using (eg) an
LVDT– Strain gauges (metal that changes resistance
when stressed)– Piezo electric materials that generate a current
when deformed
LVDT Load CellLVDT Load CellTable Force
Spring orPiston
LVDT
OuterPlatform
Strain Gauge BridgeStrain Gauge BridgeR R
R RGF LL
R R GF
ε
ε
∆ ∆= =∆
∆ = ⋅ ⋅Tension
Strain Gauges
( )
3 2
3 4 1 2
1 2 4
3
, ,
42
meas
exc
G
G
meas
meas exc
V R RV R R R R
assume R R R RR R R
VthenGF V V
ε
⎛ ⎞= −⎜ ⎟+ +⎝ ⎠= == + ∆
−=
−
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Example Load CellsExample Load Cells
Reaction torque load cell
Axial load cell
http://www.entran.com/ltoc.htm
SubminatureLoad cells
SubSub--miniature Load cells miniature Load cells
All signal conditioningand amplification integrated with the sensor Load cell bridge structure
Stefan Williams Mech 1701: Introduction to Mechatronics Slide 47
PiezoPiezo Load CellsLoad Cells• Distortion of crystal,
either quartz or BaTiO3
• Used for accurate measurement of small loads
• Come in the form of:• single axis load
washers • or multiple axis load
washers and tables
PressurePressure• Pressure measured by:
– Pitot tube and– Deformation of fixed
membrane • Deformation measured
using same methods as for force:
• Spring (manometer) • Piezo distortion• Strain gauges
Miniature
Industry IP69
High Temperature
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AccelerationAcceleration• Acceleration is also
measured via the force exerted by an accelerating mass
• Distortion of a piezo• Motion of a cantilever• Strain on mass
restraints• Accelerometers mainly
used to measure vibration
Single Axis,10,000g
Shielded for Severe environment
EMI shielded
TriTri--axial Accelerometers axial Accelerometers • Triaxial accelerometers
used in mobile systems– In high-performance cars– Inside rotating elements
of turbines– In aircraft elements
• Provide vibration information
• Provide short-term position data
Triple axisAccelerometer For racing cars
Silicon Machined AccelerometersSilicon Machined Accelerometers
Cantileverbeams
Used in eg air-bags
Silicon GyroscopesSilicon Gyroscopes• Structural arrangement
of silicon which records centrifugal acceleration and thus angular speed
• Use strain-gauge bridges and/or piezostructure to record deformations
• Multiple component elements to calibrate other accelerations
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Inertial SystemsInertial Systems• Many different types of accelerometer and
gyroscope systems• Mechanical bodies, fibre optic, etc• Together in an orthogonal arrangement of
accelerometers and gyroscopes, these comprise an inertial measurement unit (IMU)
• An IMU that is used for navigation is called an inertial navigation system (INS)
• These are widely used in aircraft and missile navigation and guidance
Aircraft
BallisticMissile
SummarySummary
• There are many types of sensors available today
• Selecting the right sensor is a critical part of the design cycle
• Requires an understanding of– Type of motion– Precision of motion– Magnitude of motion– Operating conditions
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